1. Field of the Invention
The present invention relates to a tandem-type image forming device, such as a color print engine.
2. Description of the Related Art
In a conventional tandem-type image forming device, generally, a method is employed in which a plurality of image forming units are arranged in a paper carrying direction. The image forming units include photosensitive drums. In such the method, an edge of a sheet of paper is detected for every image forming unit. Print timing is determined for each photosensitive drum based on the time of detecting an edge of a sheet of paper, the position of each of the image forming units, and the carrying speed of the sheet of paper, and printing is initiated sequentially to transfer an image to the sheet of paper.
However, the conventional tandem-type image forming device has problems to be solved as described below. That is, each of the image forming units has to be provided with a paper edge detecting sensor and a detecting circuit. Moreover, if the paper being used is shorter than that required normally for printing, the image is transferred to a place outside the paper, that is, to a carrying belt, a carrying unit such as a carrying roller, or the like. As a result, either the carrying belt is made dirty, or toner is wasteflully consumed.